In computerized industrial control systems, digital signals are applied to and received from a large number of input and output points, called I/O data points, by the computer control system to effect control over the process. Output data points are set in accordance with the applied signals and effect control functions in the process in accordance with the setting of the output data points. Input data points are set in accordance with the detected parameters of the process and the computer control system receives information as to the state of the process in accordance with the setting of these input data points. The I/O data points are organized on I/O cards mounted on I/O racks. Errors in the input and output data signals received from and applied to the I/O data points are prevented by representing the binary bits transmitted between each I/O data point and the computer control system by complementary signals, in which a binary one is represented by a high signal on a first signal line and a low signal on a second signal line and a binary zero is represented by a low signal on the first line and a high signal on the second line. When the data signals are not complementary, this condition is detected as an error. In addition, non-complementary data signals applied to an output data point will cause no change in the output data point and non-complementary signals received by the computer system from an I/O data point are not accepted by the computer as representing the state of the I/O data point.